We rely on third party partners to do so, which can be selected from your Integration dashboard page. Currently you can select between Jumio and SumSub but we plan to integrate more KYC partners so that we can cover all the more specific requirements.
If you enable Jumio, you can also provide us with the API key of your Jumio account if you have one, so that all cost associated with KYC will land into the same invoice generated by them.